Revert to preferring mmap(2) over sbrk(2) when mapping memory, due to

potential extreme contention in the kernel for multi-threaded applications
on SMP systems.

Reported by:	kris

2 files changed, 18 insertions, 13 deletions

diff --git a/lib/libc/stdlib/malloc.3 b/lib/libc/stdlib/malloc.3
index 994038d..a621a24 100644
--- a/lib/libc/stdlib/malloc.3
+++ b/lib/libc/stdlib/malloc.3
@@ -255,7 +255,7 @@ If both the
 .Dq D
 and
 .Dq M
-options are enabled, the allocator prefers the DSS over anonymous mappings,
+options are enabled, the allocator prefers anonymous mappings over the DSS,
 but allocation only fails if memory cannot be acquired via either method.
 If neither option is enabled, then the
 .Dq M
diff --git a/lib/libc/stdlib/malloc.c b/lib/libc/stdlib/malloc.c
index 00b169f..55bc413 100644
--- a/lib/libc/stdlib/malloc.c
+++ b/lib/libc/stdlib/malloc.c
@@ -1536,11 +1536,6 @@ base_pages_alloc(size_t minsize)
 {
 
 #ifdef MALLOC_DSS
-	if (opt_dss) {
-		if (base_pages_alloc_dss(minsize) == false)
-			return (false);
-	}
-
 	if (opt_mmap && minsize != 0)
 #endif
 	{
@@ -1548,6 +1543,14 @@ base_pages_alloc(size_t minsize)
 			return (false);
 	}
 
+#ifdef MALLOC_DSS
+	if (opt_dss) {
+		if (base_pages_alloc_dss(minsize) == false)
+			return (false);
+	}
+
+#endif
+
 	return (true);
 }
 
@@ -1984,6 +1987,15 @@ chunk_alloc(size_t size, bool zero)
 	assert((size & chunksize_mask) == 0);
 
 #ifdef MALLOC_DSS
+	if (opt_mmap)
+#endif
+	{
+		ret = chunk_alloc_mmap(size);
+		if (ret != NULL)
+			goto RETURN;
+	}
+
+#ifdef MALLOC_DSS
 	if (opt_dss) {
 		ret = chunk_recycle_dss(size, zero);
 		if (ret != NULL) {
@@ -1994,14 +2006,7 @@ chunk_alloc(size_t size, bool zero)
 		if (ret != NULL)
 			goto RETURN;
 	}
-
-	if (opt_mmap)
 #endif
-	{
-		ret = chunk_alloc_mmap(size);
-		if (ret != NULL)
-			goto RETURN;
-	}
 
 	/* All strategies for allocation failed. */
 	ret = NULL;